With Felisa Smith and Jim Brown, I have compared the body size distributions of mammals on four continents before and after the extinction of the megafauna at the end of the Pleistocene. We concluded that human hunting played a large role in this extinction event. On each continent, the arrival of humans occurred just prior to the extinction, whereas significant climate change occurred on only two continents coincident with the extinction. Moreover, examination of present day patterns of endangered species indicates that only large-bodied species are affected by human hunting practices (Lyons et al., 2004a). A companion paper examines the likelihood that a hyperdisease played a role in this extinction event using the spread of West Nile virus in birds in the United States. We argue that West Nile virus meets all the criteria of a hyperdisease, but is unlikely to cause an extinction with a size signature like that of the end-Pleistocene extinctions (Lyons et al., 2004b).